This invention relates generally to improvements in turbocharger compressor wheels and the like, and related compressor wheel assemblies designed for extended wheel fatigue life. More particularly, this invention relates to an improved compressor wheel assembly of the boreless hub type, wherein the compressor wheel assembly includes a spacer ring of wear resistant material for contacting adjacent turbocharger structural surfaces.
Centrifugal compressor wheels in general are well known in the art for use in turbochargers and the like, wherein the compressor wheel comprises an aerodynamically contoured array of impeller blades supported on a central wheel hub section which is mounted in turn onto a rotatable shaft for rotation therewith. In the context of a turbocharger, by way of example, the wheel hub section conventionally includes a central axial bore through which the shaft extends, and a threaded nut is fastened over the shaft at a nose end of the compressor wheel to hold the hub section tightly against a shaft shoulder or other diametrically enlarged structure such as a thrust bearing unit on the shaft. The rotatable turbocharger shaft thereby drives the compressor wheel in a direction such that the contoured blades axially draw in air for discharge radially outwardly at an elevated pressure into a volute-shaped chamber of a compressor housing. The pressurized air is then supplied from the housing to the air intake manifold of a combustion engine for mixture and combustion with fuel, all in a well known manner.
In recent years, improvements in compressor technology and design have resulted in progressive increases in compressor efficiency and flow range, together with more rapid transient response characteristics. For example, compressor wheels for turbochargers are known wherein the contoured impeller blades exhibit compound and highly complex curvatures designed for optimum operational efficiency and flow range. Such complex blade shape is most advantageously and economically obtained by a casting process wherein the wheel hub section and blades are integrally formed desirably from a lightweight material, such as aluminum or aluminum alloy, chosen for its relatively low rotational inertia consistent with rapid wheel accelerations and decelerations during transient engine operating conditions. Cast compressor wheels of this general type, however, have occasionally exhibited a relatively short, finite fatigue life resulting in undesired occurrences of wheel fatigue failure during operation. More specifically, when the compressor wheel is rotated at very high speeds, the cast aluminum material is subjected to relatively high centrifugal loading in a tangential direction particularly near the center, wherein the hub region of the compressor wheel must support the rapidly rotating wheel mass by the action of radial and tangential stresses. The impact of these stresses is especially severe when the compressor wheel is operated at a relatively high speed, rapid speed cycle environment as encountered, for example, in many modern turbocharger installations. Unfortunately, as previously described, the hub region of the compressor wheel normally includes a major void in the form of the central bore for receiving the rotatable shaft, wherein the central bore acts a major stress riser rendering the wheel highly susceptible to fatigue failure in the hub region. This fatigue failure problem is compounded by the presence of any metallurgical imperfections, such as dross, voids, and/or inclusions which sometimes occur inherently during the casting process and tend to congregate in the hub region of the compressor wheel.
Improvements in cast compressor wheels have been proposed in the form of a compressor wheel having a boreless hub section to eliminate the stress riser site provided by the conventional shaft bore. While such alternative wheel designs theoretically provide extended fatigue life, a variety of significant design problems have been encountered.
For example, boreless hub compressor wheels have normally included an internally threaded sleeve extending coaxially from the boreless hub for attachment to the threaded end of a rotatable turbocharger shaft. This threaded sleeve is formed to avoid any internal bore within the centrifugal compressor wheel along the axial span between the nose end and a plane corresponding with the maximum diameter of the wheel. To avoid undesired axial extension of the turbocharger or the like, the threaded sleeve is fitted through an adjacent bore or opening in a compressor backplate for direct attachment to the rotatable shaft, with one or more seal rings being carried about the sleeve for sealing contact with the backplate. However, assembly of the various components, including threaded attachment of the sleeve to the shaft while seating one or more seal rings within the backplate opening, can be extremely difficult. Moreover, when the threaded sleeve is cast integrally with the compressor wheel from the same lightweight material, the sleeve possesses inadequate structural hardness to support the seal ring or rings without undue wear, or to engage adjacent structural surfaces such as a thrust bearing unit without undue wear.
Alternative boreless compressor wheel designs have envisioned welded attachment of a separate sleeve formed from a suitable hard bearing material onto a boreless hub wheel, as disclosed in U.S. Pat. No. 4,705,463. While this approach reduces wear attributable to seal rings and/or contact with thrust bearing components, the concept encounters significant balancing problems and may undesirably introduce new stress riser zones within the wheel hub region as a result of exposure to welding heat. Other boreless wheel configurations have utilized modified thrust bearing components with axially elongated spacer sleeves projecting into the backplate opening for supporting one or more seal rings, as disclosed in U.S. Ser. No. 428,927, filed Oct. 30, 1989.
The present invention provides further alternative compressor wheel assembly designs wherein wear resistant components and/or surfaces are assembled onto a mounting sleeve of a boreless hub compressor wheel, and further wherein the wear resistant components are strategically positioned to minimize and/or eliminate wear of the cast wheel structure. Moreover, the wear resistant components are designed for relatively simple mounting onto the cast wheel structure, without requiring welding processes.